Growth of the heart in fetal and neonatal animals involves muscle cell hypertrophy and hyperplasia and is associated with an increased tissue content of RNA. The extent to which elevated capacity for protein synthesis, as assessed by ribosome content, or improved efficiency with which the existing ribosomes, mRNA, and initiation and elongation factors are used has not been investigated in the immature heart. Preliminary data obtained in this laboratory suggest that increased RNA content is one of the major factors accounting for the faster rate of overall protein synthesis in the piglet heart. The hypothesis that is to be tested is that synthesis of cytoplasmic ribosomes depends upon the balance between rates of pre-rRNA synthesis and processing and between rates of synthesis and degradation of ribosomal proteins. A rigorous determination of synthetic rates for whole heart protein and ribosomal protein and for rRNA are essential for an understanding of mechanisms of cardiac growth. Rates of synthesis of 28S and 18S RNA in perfused hearts and heart muscle cells will be based on the specific activity of the immediate precursor, pre-rRNA and will involve labelling with (3H)uridine or (methyl-3H) methionine. Rates of synthesis of pre-rRNA will be based on the incorporation of radioactivity into pre-rRNA, the specific activity of UTP during the period when labelling of pre-rRNA is rising and the steady-state relationship between specific activities of UTP in the tissue and UMP in pre-rRNA. Rates of synthesis of ribosomal proteins will be based on the specific activity of phenylalanyl-tRNA and rates of incorporation of radioactivity into individual ribosomal proteins or ribosomal core proteins. The hearts of developing pigs from animals at 0.9 gestation to 10 days after birth will be compared. Rates of RNA and protein synthesis will be assessed in the right and left ventricular free walls and septum. The left ventricular free wall grows at a rate 3.5 times greater than the right ventricular free wall over these age ranges. Hearts will be perfused as Langendorff and working preparations with buffer that simulates the plasma content of glucose, lactate, insulin, and amino acids. These studies will allow for rigorous estimates of rates of synthesis and processing of rRNA, of synthesis of whole heart protein and of synthesis and incorporation of r-protein into ribosomal subunits. As a result, the early events during rapid growth of immature hearts will be identified and the mechanism of normal myocardial growth will be more completely understood.